This invention relates to a filter system for removing particulates from the exhaust gases of an internal combustion engine, in particular a diesel engine, having at least one filter member formed by honeycomb filter channels and made of a porous filter material, whereby electrical resistance heating elements, which are connected via a lead-in and a lead-out to a power supply are mounted in the region of the intake openings of the filter channels, open on the gas intake side.
In order to reduce the emission of particulates, particularly in diesel engines, various types of exhaust aftertreatment systems are known. Usually they comprise filter systems which retain and collect the particulates in the exhaust gas. The particulates, retained in the filter, may lead to an increase in the flow resistance in the exhaust system so that the exhaust back pressure of the engine increases. This in turn leads to an increase in fuel consumption and in extreme cases to engine failure. Therefore, it is necessary to remove the particulates deposited in the filter, for example, by means of oxidation at high temperatures.
Honeycomb filters of a porous ceramic material have proven themselves to be suitable as a filter member for retaining the soot particles. These honeycomb filters are formed by a plurality of parallel filter channels, which are closed alternately on the gas inlet side and the gas discharge side so that the exhaust gases must flow through the porous filter walls and thus the particulates are deposited on the walls of the filter channels. The filter can be regenerated by incinerating the accumulated particulates.
The temperature required to ignite the soot particles is not attained sufficiently often so that regeneration is not assured. Automatic regeneration can be attained by a supply of additional energy. An energy-efficient regeneration can be attained if in the inlet region of the filter channels the particulates, deposited in the filter member, are ignited punctually by means of a short-term supply of energy. The energy that is then released by the initial incineration of the particulates can then lead to a self-supporting incineration of the particulates in the filter member. The layer of particulates can be ignited by means of looped resistance wires positioned in the openings of the filter channels. In order to facilitate complete regeneration, the loops of the conductor must be inserted into as many filter channels of the honeycomb filter as possible. The number of filter channels, which can be provided with loops, is limited by the electrical resistance of the conductor.
With a 12 V. supply voltage, which is common in vehicles, the length of the conductor ranges from 15 to 25 cm., of which 10 to 15 loops can be bent. Ceramic honeycomb filters have approximately 1,000 channels, which have to be heated. In order to heat the filter as completely as possible, a large number of individual heating wires bent in the shape of loops are inserted parallel and connected. In order to regenerate the entire filter simultaneously, a large quantity of heat is required, which cannot be supplied by the electrical wiring system of the vehicle. Thus, the quantity of heat can be supplied only by sequential regeneration of individual subregions of the filter. An example of this is known from U.S. Pat. No. 4,427,418.
The loop-shaped bent conductors must be interconnected into small groups to facilitate carrying out sequential regeneration. The individual groups are electrically separated from one another and connected to the supply voltage of the vehicle in such a manner that they can be switched on independently of one another. The distance between the individual connections, which must be electrically insulated from one another, is very small due to the small cross-section of the filter channels of approximately 2.times.2 mm. Any contact between the individual connections would result in a short-circuit while the vehicle was operating, or several areas would be energized with a power consumption that is too high for the electrical system of the vehicle. If heating wires migrate, it can also result in a bridging of individual loops. The result is that the electrical resistance of the conductor drops, whereby the temperature of the conductors rises and the wires can burn through.